Inductively coupled plasma (ICP) process reactors generally form plasmas by inducing current in a process gas disposed within the process chamber via one or more inductive coils disposed outside of the process chamber. The inductive coils may be disposed externally and separated electrically from the chamber by, for example, a dielectric lid. When radio frequency (RF) current is fed to the inductive coils via an RF feed structure from an RF power supply, an inductively coupled plasma can be formed inside the chamber from an electric field generated by the inductive coils.
In some reactor designs, the reactor may be configured to have concentric inner and outer inductive coils. RF power from an RF power source may be split between the two coils via a current divider/variable capacitor, or the like. In some reactors, the power from a RF power source may be coupled through a dynamically tuned matching network (also referred to as a match unit) to an antenna or electrode within the reactor. The RF power is coupled from the antenna or electrode to process gases within the reactor to form a plasma that is used for the etching process. The matching network ensures that the output of the RF source is efficiently coupled to the plasma to maximize the amount of energy coupled to the plasma (e.g., referred to as tuning the RF power delivery). Tuning refers to the process of varying (e.g., tuning) the impedance of the electrical pathway seen by the RF source (i.e., plasma impedance+chamber impedance+matching network impedance) in order to minimize power reflected back to the RF power source from the plasma and maximize efficient coupling of power from the RF power source to the plasma.
Existing reactor designs focus on ways to rapidly minimize the amount of reflected power (i.e., controlling the reflected power) to tune the system. By contrast the inventor has recognized that by controlling the current ratio between the two coils, rather than focusing on reflected power, more effective tuning can be achieved in certain situations. However, in existing solutions involving tuning using current ratio control, the limits of the current ratio are fixed for each operating mode (standard/forward current mode and reverse mode).
Accordingly, the inventors have devised a plasma process method and apparatus to better control current ratio.